The present invention relates to apparatus and methods for chemical mechanical polishing a substrate.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface can present problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface. In addition, plaranization is needed when polishing back a filler layer, e.g., when filling trenches in a dielectric layer with metal.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a polishing pad, e.g., a circular pad or linear belt, that moves relative to the substrate. The polishing pad may be either a xe2x80x9cstandardxe2x80x9d pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
During CMP operations, the polishing pad needs to be replaced periodically. For a fixed-abrasive pad, the substrate wears away the containment media to expose the embedded abrasive particles. Thus, the fixed-abrasive pad is gradually consumed by the polishing process. After a sufficient number of polishing runs the fixed-abrasive pad needs to be replaced. For a standard pad, the substrate thermally and mechanically damages the polishing pad and causes the pad""s surface to become smoother and less abrasive. Therefore, standard pads must be periodically xe2x80x9cconditionedxe2x80x9d to restore a roughened texture to their surface. After a sufficient number of conditioning operations, the conditioning process consumes the pad or the pad is unable to be properly conditioned. The pad must then be replaced.
One problem encountered in the CMP process is difficulty in replacing the polishing pad. The polishing pad may be attached to the platen surface with an adhesive. Significant physical effort is often required to peel the polishing pad away from the platen surface. The adhesive then must be removed from the platen surface by scraping and washing with a solvent. A new polishing pad can then be adhesively attached to the clean surface of the platen. While this. is happening, the platen is not available for the polishing of substrates, resulting in a decrease in polishing throughput.
In one aspect, the invention is directed to a chemical mechanical polishing apparatus that has a rotatable platen, a feed roller located in a cavity in the platen, a take-up roller, and a generally linear polishing sheet releasably secured to the platen to rotate with the platen. The polishing sheet has an exposed portion extending over a top surface of the platen for polishing the substrate, an unused portion wound around the feed roller, and a used portion wound around the take-up roller. The apparatus also has a drive mechanism to incrementally advance the polishing sheet in a linear direction across the top surface of the platen, and a gas source that directs a purge gas into the cavity containing the feed roller.
Implementations of the invention may include one or more of the following features. The purge gas may maintain the cavity at a pressure greater than atmospheric pressure. The purge gas may be nitrogen. The apparatus may have a door to the cavity pivotally attached to the platen.
In another aspect, the invention is directed to a method of operating a chemical mechanical polishing apparatus. In the method, a generally linear polishing sheet is positioned with an unused portion wound around a feed roller in a cavity of a rotatable platen, an exposed portion extending over a top surface of the platen, and a used portion wound around a take-up roller. A purge gas is directed into the cavity containing the feed roller. The platen rotates, and the polishing sheet advances in a linear direction across the top surface of the platen.
Implementations of the invention may include one or more of the following features. The purge gas may maintain the cavity at a pressure greater than atmospheric pressure. The purge gas may be nitrogen.
Potential advantages of the invention may include the following. Contamination on the unused portion of the polishing sheet may be reduced, thereby decreasing defects and increasing yield.
Other features and advantages will be apparent from the following description, including the drawings and claims.